1. Field of the Invention
The present invention relates to a lenticular lens sheet to be used as the so-called rear projection screen for television which displays an image projected thereon from behind for observation from the front side thereof and, more particularly, to a lenticular lens sheet whose optical characteristics can be changed when used in combination with a Fresnel lens to form a rear projection screen.
2. Description of the Related Art
A lenticular lens sheet to be used as a screen for a rear projection television set, in which an image is projected on the screen from the back side of the screen and the image displayed on the screen is watched from the front side of the screen, is manufactured by the extrusion molding the press forming of a resin material prepared by kneading a mixture of a transparent or translucent base resin and a diffusing agent.
Such a lenticular lens sheet has a light entrance surface provided with linear lenticular elements having a cross section of a shape resembling a portion of a circle or an ellipse, and a light exit surface provided with a lens formed by alternately arranging shading black stripes and lenticular elements having a cross section of a shape resembling a portion of a circle or an ellipse. The lenticular elements of the light entrance surface and those of the light exit surface are formed integrally.
When manufacturing the conventional lenticular lens, the shape of the lenticular elements is designed according to desired optical characteristics of the lenticular lens, a die for molding the lenticular lens is fabricated on the basis of optical design data on the lenticular elements, and a resin is molded in the die to obtain the lenticular lens. However, it is very difficult to fabricate the die accurately conforming to the optical design data by state-of-the-art die fabricating techniques. For example, the sectional shape of the lenticular elements formed in the light entrance surface of the lenticular lens sheet is a half or a quarter of a circle of a radius in the range of 0.1 to 0.2 mm or a half of an ellipse. It is hardly possible to form accurately lenticular elements of a shape more complicated and minuter than the lenticular elements of the foregoing shape and size by state-of-the-art die fabricating techniques. When it is desired to obtain a lenticular lens sheet having further complicated optical characteristics, available means for adjusting the optical characteristics of the lenticular lens sheet have been only changing the diffusing agent contained in the resin material, changing the diffusing agent content of the resin base and/or changing the dimensions of the cross section of the lenticular elements which is a portion of a circle or an ellipse.
Some lenticular lens sheet is formed by putting together two lenticular lens sheets having linear lenticular elements with their lenticular elements extended perpendicularly to each other.
The rear projection screen using the conventional lenticular lens sheet is provided with linear lenticular elements. Therefore, the rear projection screen has uniform light exit characteristic for incident light rays falling on the light entrance surface at different incident angles, and the distribution of outgoing light rays is controlled in a limit range. Such problems may be solved by using a lenticular lens sheet formed by putting together two lenticular lens sheets having linear lenticular elements with their lenticular elements extended perpendicularly to each other, which unavoidably entails a light loss and increase in material cost because two lenticular lens sheets are used in two layers.
The lenticular lens sheet may be provided with lenses similar to a compound eye. However, it is very difficult for state-of-the-art die fabricating techniques, which form linear grooves of a circular cross section in a workpiece to form a die with the tip of a cutting tool, to fabricate a die for forming a lenticular lens sheet having lenticular elements having surfaces provided with minute ridges and furrows far smaller than those of the conventional lenticular lens sheet so as to conform accurately to optical design data. Use of an engraving process and a grinding process for fabricating a die will increases the costs of the die greatly.